144 DISCOVERY CH. 



died at that place in 1382. In a translation of the four 

 books of Aristotle, Oresme commented upon Aristotle's 

 contentions for an immovable, central earth, gave 

 numerous reasons and arguments against this hypothesis, 

 and showed it to be unsound. The work in which this 

 was done was never published, though several manuscript 

 copies exist. Probably other thoughtful inquirers had 

 the same dissatisfaction with the theory of a fixed 

 central earth, but were unwilling to publish any criticism 

 of what was then the orthodox view. It was perhaps 

 fortunate for Copernicus himself that the work in which 

 he finally dethroned the earth from the position it had 

 held for so long and, following Aristarchus, made the 

 sun the centre of our system, was not published to the 

 world until he was on his death-bed in 1543. 



In the plan of the solar system conceived by Coper- 

 nicus, the planets Mercury and Venus are nearer the 

 sun than is the earth ; and in the. course of their revolu- 

 tions around the body to which they owe their light 

 they must, therefore, exhibit phases like those through 

 which the moon passes monthly. Copernicus knew 

 that these phases were a consequence of his theory, 

 and predicted that they would be found to exist. One 

 of the first uses to which Galileo put his telescope in 

 1610 was the observation of Venus, and he wrote a full 

 account of the changes seen by him in the appearance 

 of the planet, from the fully-illuminated disc to the 

 crescent form and then through the half-moon aspect 

 to full again. The discovery of these phases provided 

 an unanswerable argument for the Copernican theory. 



From the observation of these wonderful phenomena we are 

 supplied with a determination most conclusive, and appealing 

 to the evidence of our senses, of two very important problems, 

 which up to this day were discussed by the greatest intellects 

 with different conclusions. One is that the planets are bodies 



